Brushless DC motors without permanent magnets often are interchangeably referred to as switched reluctance (SR) or variable reluctance (VR) motors. Reference to a VR motor herein is intended to include both terminologies. A VR motor has two sets of salient poles, one set on the stator which has phase windings around the poles and another set on the rotor which has no windings. The stator phase windings are sequentially energized with current pulses to rotate the rotor which is connected to a shaft output. The stator phase windings are sequenced, or commutated, by signals from a rotor position sensor. The rotor position sensing means may comprise optical sensors or magnetic sensors of the Hall effect type. The sensors typically are mounted in fixed position on the stator or motor housing adjacent the path of rotation of the rotor, and the sensed means are fixed for rotation with the rotor.
In a typical three-phase, VR motor, three Hall effect sensors may be located 120.degree. arcuately apart, centered about the rotor shaft, and are fixed either directly to the stator or to some fixture which locates them according to some known relationship with respect to the stator. A magnetic ring with four North regions and four South regions alternating in 45.degree. radial arcs of the ring are attached to the rotor or rotor shaft and serve as sensed means so that when the rotor rotates, sensor output signals can be used to directly commutate, i.e., cut on and off, the current to each of the motor phase windings as they locate each and every pole alignment.
VR motors have been proposed for driving the individual spindle assemblies of a textile yarn ring spinning frame. In such spindle assemblies, the rotor of the motor is mounted on the spindle shaft which supportably rotates a yarn collection member, such as a bobbin, during the spinning operation. A ring rail with ring and traveler reciprocates vertically along the support bobbin to wind the yarn package. The lower end of the spindle support shaft is supported for rotation in a bolster section which has an outer housing mounted in fixed position to a spindle assembly support rail of the spinning frame. The stator of the VR motor is disposed in surrounding relation to the rotor and is mounted in fixed position in a housing supportably attached in suitable manner to the bolster housing or support rail of the ring spinning frame.
In a paper entitled "Four-Quadrant Brushless Reluctance Motor Drive" by T. J. E. Miller et al., presented at IEE Conference on Power Electronics and Variable Speed Drives, London, July 1988, there is suggested that the simplest possible bi-directional scheme for a three-phase motor uses only two sensors (Hall or optical). A paper entitled "A New Control IC For Switched Reluctance Motor Drives" by T. J. E. Miller et al., presented at IEE Power Electronics and Variable Speed Drive Conference, London, July 1990, states that the internal commutation logic of a VR motor can operate with shaft position signals derived from either 3 or 2 Hall or optical sensors.
The use of three Hall effect sensors to detect and commutate a three-phase motor of the VR type, as in the prior art, works well; however, it is advantageous to use as minimum a number of sensors as possible to decrease the expense of the system as well as decrease the mean-time-to-failure for a system that results from the use of additional sensing means.